In general, a purpose of battery internal condition estimating methods is to estimate a charging rate (SOC: State Of Charge) of a battery. An example method of estimating an SOC value is to total integrated values of charge/discharge currents with reference to a relationship between an open circuit voltage (OCV: Open Circuit Voltage) that is a voltage when no current is flowing and the SOC, thereby estimating the SOC.
When, however, charging/discharging is always performed like an application of an output fluctuation suppression for wind power generation, it is difficult to obtain the OCV, and when the charging/discharging continues for a long time, minute current integration errors are accumulated. Hence, eventually, the charging/discharging should be purposefully stopped to reset such errors and to obtain the OCV. A method that can cope with this problem is an SOC estimating method that utilizes a Karman filter.
According to the SOC estimating method that utilizes a Karman filter, a current actual measurement value and a voltage actual measurement value detected by respective detectors connected with a battery are obtained, and based on those measurement values, a battery equivalent circuit model that includes resistances and capacitors is created using the Karman filter. Next, the OCV is estimated based on this battery equivalent circuit model, and the SOC is calculated based on relationship data between the OCV estimated value and the SOC of the batteries. By sequentially applying a capacitor capacitance obtained from the OCV estimated value obtained at the last sampling time, a highly precise SOC is obtainable.
According to the above battery internal condition estimating method, however, the SOC can be estimated when the battery is a single unit, but when this method is applied to estimate the SOC (hereinafter, referred to as a system SOC) of an electricity storage system that has plural batteries connected by multiple parallel-series connection scheme, it is unclear which voltage of the battery should be applied for the estimation.
In addition, a method of estimating respective SOCs of all batteries and adopting the average value thereof as a system SOC is possible, but in the case of a large-scale electricity storage system, it is necessary to perform calculation battery by battery although the number of batteries is quite large, and thus the amount of calculation may increase.